1. Field of the Invention
This invention relates to an image formining apparatus, such as a copier, a recorder, e.g., a printer, a display or the like, for forming an image by applying a series of image forming processes including a charging (inclusive of charge removing) process for a reusable image carrying member made of a photosensitive material, a dielectric material or the like in an electrophotographic method, a static recording method and the like.
More particularly, this invention relates to an image forming apparatus in which at least an image carrying member and a charging unit for performing charging processing for the surface of the image carrying member among a series of image forming process units are combined in the form of a process cartridge to be detachably mounted on the main body of the image forming apparatus.
Further, this invention relates to an image forming apparatus which uses a charging unit for charging an image carrying member contacting the image carrying member as the charging unit.
2. Description of the Related Art
An image forming apparatus having a detachable process cartridge has mostly been used as the configuration for a convenient and small image forming apparatus suitable for personal use.
For example, in a copier or a printer which utilizes a transfer-type electrophotographic process, a photosensitive drum as an image carrying member, a charging unit for uniformly charging the surface of the drum, a developing unit for developing a latent image formed on the surface of the drum, a cleaning unit for cleaning the surface of the drum after transfer of the image, and the like are combined in the form of a process cartridge which even an inexperienced user can easily detach from the main body of the apparatus and install a replacement unit. Such a detachable process cartridge has, for example, the following advantages. When the detachable process cartridge has reached its durability limit, such as the useful life of the photosensitive drum, consumption of a developer previously housed in the developing unit, and the like, it is replaced with a new cartridge. The image forming apparatus thus becomes maintenance-free with respect to the cartridge. Hence, service capability is improved. Furthermore, by exchanging various kinds of process cartridges having different developing colors, it is possible to change the color of an output image for single color printing, or to form a multicolor synthesized image for multicolor printing.
FIG. 1 shows the schematic configuration of a laser-beam printer as an example of the image forming apparatus having a detachable process cartridge.
In FIG. 1, there is shown an outer case 1 of the apparatus. A photosensitive drum 2, serving as an image carrying member is rotatably driven with a predetermined circumferential speed in the clockwise direction indicated by an arrow. While it is being rotated, the photosensitive drum 2 is subjected to either positive or negative uniform charging by a corona discharge unit 40 as a charging processing means, laser-beam scanning exposure L of the information of an object image by a laser-beam scanner 5, and development (reversal development) by a developing unit 6. Thus, a toner image of the object image is sequentially formed on the surface of the drum 2. A transfer sheet material P is fed one by one from a paper cassette 14 into the apparatus by a paper feeding roller 15. The sheet material P is conveyed to a transfer portion between the drum 2 and a transfer unit 7 by a pair of registration rollers 16 in synchronization with the rotation of the photosensitive drum 2. The toner image on the drum 2 is then transferred to the surface of the conveyed sheet material P.
The sheet material P on which the toner image has been transferred while passing through the transfer portion is then separated from the surface of the drum 2, and is introduced into a fixing unit 11 by a conveying unit 10. The image is fixed in the fixing unit 11, and the sheet material having the fixed image is output onto a paper discharging tray 13 outside the apparatus as a printed matter by a discharging roller 12.
After the transfer of the toner image, the surface of the drum 2 is subjected to removal of any attached contaminants, such as residual toner and the like, by a cleaning unit 8, erasure of electric residual memory by preexposure processing, i.e., light illumination upon the entire surface by an erasure lamp 9, and is then ready for another image formation. A mirror 5a reflects the laser beam output from the laser-beam scanner 5 in the direction of the surface of the photosensitive drum 2.
In the apparatus of the present example, the four process units, i.e., the photosensitive drum 2, the corona discharge unit 40, the developing unit 6 and the cleaning unit 8, are configured as a process cartridge 20. The cartridge 20 is detachably mounted on a predetermined position within the main body of the apparatus by releasing the apparatus by means of an appropriate opening/closing method for the apparatus, such as a method of releasing the front door and upper body of the apparatus or the like. When the cartridge 20 is properly mounted within the main body of the apparatus, the units within the cartridge 20 and the units within the main body of the apparatus are mechanically and electrically coupled with one another. The units 2, 6, 8 and 40 are incorporated within an outer housing, i.e., a cartridge frame 21 of the cartridge combined in a predetermined relationship. An opening 22 for exposure by an incident laser beam and an opening 23 for preexposure by the incident erasure light beam are provided at respective predetermined positions on the upper surface of the cartridge housing 21.
As a unit for charging the photosensitive drum 2 for readiness as an image carrying member, the corona discharge unit 40 having a corona discharging wire is generally used. However, such a corona discharge unit has at least the following four problems:
1) It is necessary to apply a high voltage of 4-8 kV (kilovolts) to the wire in order to obtain the surface potential of 500-700 V (volts) on the photosensitive body. The distance between the wire and the electrodes must be large in order to prevent leakage to the electrodes and to the main body, and the like. Consequently, the discharging unit itself becomes large in size. Furthermore, it is necessary to use a covered cable to provide a high level of insulation.
2) The majority of the discharge current from the wire flows to a shielding electrode. The corona current flowing to the side of the photosensitive body as a member to be charged represents only a few percent of the total discharge current.
3) Discharge products, such as ozone and the like, are generated by the corona discharge. Accordingly, oxidation of components in the apparatus and image blurring, shading due to degradation of the surface of the photosensitive body by ozone are easily produced. This phenomenon is particularly pronounced in highly humid environment. by ozone are easily produced. In addition, it is necessary to provide a filter for absorbing and decomposing ozone, and a fan as an airflow generation means for the filter to prevent ozone from coming in contact with humans.
4) In order to increase discharge efficiency, a discharge wire having a relatively large diameter is used. In general, a wire having a diameter of 60-100 .mu.m is preferred. A high electric field formed on the surface of the wire attracts fine dust within the apparatus. Therefore, the surface of the wire becomes contaminated. The contamination of the wire tends to produce an uneven discharge, which results in unevenness in the formed image. Hence, it is necessary to frequently clean the wire and the inside of the discharge unit.
Particularly troublesome is an image forming apparatus in which a process cartridge 20 including at least an image carrying member and a corona discharge unit 40 as a charging means for charging the image carrying member incorporated within a closed cartridge housing 21. Hence, airflow within the cartridge housing 21 is obstructed, and discharge products, such as ozone and the like, tend to be present in high density between the inner and outer peripheral portions of the discharge unit 40 and within the entire cartridge, in general. Accordingly, the above-described problem 3) becomes particularly serious.
Heretofore, it has been intended to prevent the problems of image blurring, shading and the like caused by discharge products by the following approach. By making the opening 22 for exposure in the cartridge housing 21 an air inlet and the opening 23 for preexposure an air outlet, a vacuum system is provided so that positive airflow A in the circumferential direction of the drum is drawn in from outside the housing 21 through the opening 22 then passes through the inside of the housing 21 between a facing gap portion between the photosensitive drum 2 and the discharge unit 40 through the opening 23 to the outside of the housing. Thus, the air containing discharge products, such as ozone and the like, which are produced particularly within and around the discharge unit 40 and the air within the housing 21 are positively exhausted to the outside. U.S. Pat. No. 4,540,268 issued to Toyono, et al. on Sept. 10, 1985, for a Process Kit and Image Forming Apparatus Using Such Kit discloses a kit 17 having ventilation slits positioned on either side of a discharger.
On the other hand, there has recently been studied the utilization of a contact charging means in place of a corona discharge unit but such has various problems as a charging means.
More specifically, the surface of a photosensitive body is charged to a predetermined potential by contacting the surface of the photosensitive body as a body to be charged with a conductive member, i.e., a conductive member for maintaining potential, such as a brush made of conductive fibers, a conductive roller or the like, to which a D.C. voltage of about 1 kV is applied.
FIG. 2 shows a cartridge in which a charging roller 4 as a contact charging member is disposed contacting the surface of the photosensitive drum 2 in place of the corona discharge unit 40 as a charging means in the process cartridge 20 of FIG. 1 apparatus.
The contact charging means has the advantages that it can function with a voltage which is lower than the voltage needed for the corona discharge unit. Consequently, the amount of discharge products, such as ozone and the like, is small. Nevertheless, in the case of using the process cartridge, although the amount of discharge products is small, discharge products the amount of which finally causes image blurring, shading and the like are eventually accumulated. If the air within the cartridge housing is not ventilated, the discharge products accumulate around the charging member and in the space within the housing while charging operations are repeatedly performed.
However, ventilation means in the case of using the corona discharge unit as the charging means as described above is not practically effective as a ventilation means within the cartridge. That is, in the case of using a contact charging means, since the charging roller 4 as the charging means is disposed so as to contact the surface of the photosensitive drum 2 in the direction of the drum's generatrix. Therefore, a gap portion for ventilation does not exist between the photosensitive drum 2 and the charging roller 4 as in the case of using the corona discharge unit 40. (Actually, as will be discussed later a "microscopic" gap does not exist therebetween.) Accordingly, even if a ventilation means for introducing external air from the opening 22 and exhausting the air from the opening 23 is provided within the cartridge housing 21, sufficient airflow is not produced at portions "a" and "b" (in FIG. 2). The portions are near the charging roller situated at the upstream and downstream sides relative to the rotation of the drum 2 from the portion where the charging roller contacts the photosensitive drum 2. Hence, discharge products tend to be accumulated and remain at these portions with the passage of time, causing image blurring, shading, and the like.